From a point of view of protection of a driver and/or a passenger in a car, an air bag apparatus which is a passenger protection apparatus has conventionally widely been used. The air bag apparatus is equipped for the purpose of protecting a driver and/or a passenger against shock caused at the time of collision of a vehicle, and it receives a body of a driver or a passenger with an air bag serving as a cushion, as the air bag is expanded and developed instantaneously at the time of collision of the vehicle.
The gas generator is equipment which is incorporated in this air bag apparatus, an igniter therein being ignited in response to power feed through a control unit at the time of collision of a vehicle to thereby burn a gas generating agent with flame caused by the igniter and instantaneously generate a large amount of gas, and thus expands and develops an air bag.
Gas generators of various structures are available. A disc-type gas generator in a substantially short columnar shape relatively great in outer diameter is available as a gas generator suitably used for an air bag apparatus on a driver's seat side or an air bag apparatus on a passenger's seat side, and a cylinder-type gas generator in a substantially long columnar shape relatively small in outer diameter is available as a gas generator suitably used for a side air bag apparatus, a curtain air bag apparatus, and a knee air bag apparatus.
As described in Japanese Patent Laying-Open No. 1-172047 (PTD 1), for example, a disc-type gas generator including a housing including an upper shell (a closure shell) and a lower shell (an initiator shell) and a filter provided inside the housing, a plurality of gas discharge openings for discharging gas through the filter being provided in a circumferential direction of the upper shell, has been known as the disc-type gas generator.
It is important for a gas generator to burn a gas generating agent continually in a stable manner at the time of activation. In order to burn the gas generating agent continually in a stable manner, the gas generating agent should be placed in a prescribed high-pressure environment. Therefore, the gas generator is designed to narrow a size of a plurality of gas discharge openings provided in the housing to a desired size so that a pressure in a space in a housing is raised to a considerable level at the time of activation.
Output characteristics of the gas generator, however, are affected by an ambient environment in which the gas generator is placed, and particularly dependent on an environmental temperature. The output characteristics tend to be enhanced in a high-temperature environment and weakened in a low-temperature environment. In the high-temperature environment, gas is discharged earlier and more strongly, and in the low-temperature environment, the gas is discharged more slowly and weakly. Therefore, in particular in the low-temperature environment, significant drop in pressure in the housing due to opening of the gas discharge openings tends to occur, continual burning of the gas generating agent may be impeded, and gas output may be insufficient.
In order to lessen difference in gas output performance due to an environmental temperature, for example, International Publication WO2015/163290 (PTD 2) discloses a disc-type gas generator constructed to include a plurality of gas discharge openings different in opening pressure provided in a housing. In the gas generator constructed as such, the plurality of gas discharge openings are opened stepwise with increase in pressure in a space inside the housing.
Therefore, as compared with a gas generator constructed such that all gas discharge openings are opened in unison with increase in pressure in a space inside a housing, significant drop in increase in internal pressure in particular in a low-temperature environment can be prevented. Therefore, a gas generating agent can burn continually in any temperature environment from a high-temperature environment to a low-temperature environment, and consequently a difference in gas output performance attributed to an environmental temperature can be lessened.
FIGS. 10 to 12 of PTD 2 disclose a disc-type gas generator constructed such that a plurality of gas discharge openings are opened in three stages with increase in pressure in the space inside the housing at the time of activation by providing in a circumferential wall portion of the housing, the plurality of gas discharge openings of which opening pressures are set in three stages. When general specifications required of a disc-type gas generator are taken into consideration, the gas generator is preferably set such that the plurality of gas discharge openings are opened in three stages.